A program is described for measuring the growth and development of physical working capacity in children 6, 8, 10, 12, 14 and 16 years of age in upper and lower socioeconomic groups in urban and rural populations of the Valle del Cauca in Colombia. The development of work capacity will be related to the nutritional status of the children. The prevalence of protein-calorie malnutrition among the lower socioeconomic urban and rural populations and its absence in the upper socioeconomic groups will allow for recruitment of ample numbers of subjects in each of the experimental groups. The research is directed towards qualifying the impact of undernutrition on the development of the capacity to do work and therefore to produce and contribute to the economic productivity of the country. The maximal ability to consume oxygen, as determined by the progressive treadmill procedure, will be utilized as the measure of physical working capacity. Nutritional status will be assessed by standard anthropometric measurements and blood analysis for hematocrit, hemoglobin, serum albumin and total proteins.